Toner printing is one common method for forming an image on a receiver. In toner printing, toner particles are patterned to form toner images and these toner images are transferred and fused to the receiver to form a print. Toner particles typically take the form of small particles of a transparent binder material having a colorant such as a pigment or a dye therein. The colorant reflects color forming wavelengths of light while absorbing other wavelengths and causes the toner particle to appear to have a particular color.
To form multi-color images, a plurality of toner images are transferred in register and collectively fused to a receiver to form a composite toner image. Typically each toner image is formed using a different toner having a different color of reflective colorant. The colors are selected so that toners can be combined to form specific ranges colors on a receiver according to a subtractive color model in which increases in color gamut are made possible by subtracting a greater proportion of an ambient light.
Prints made according to the subtractive color model therefore have a lowest density at unprinted areas and highest density where there is the most toner. The density difference between the brightest point in a toner print and the darkest point in a toner print is known as the dynamic range of the print. It is generally preferred to provide prints having a high dynamic range as this expands the color gamut of the prints. However, it is difficult to provide high dynamic range in a toner print on a reflective receiver using toners having reflective colorants.
For example, it has proven to be challenging to extend the dynamic range of a toner print by providing for additional high density levels. This requires that a printer be capable of providing one or more printable levels that are have a visibly higher density than preceding ones. This approach becomes increasingly challenging with each new level of additional density because absorbing ambient light to an extent that is visibly differentiable from the density of a very high density portion of an image requires a substantial amount of toner. The incremental difference in toner required to create each new level of higher density required to increase the dynamic range of the print increases significantly with each new level of higher density and quickly becomes inefficient. Further, this approach can create artifacts in terms of continuing depositing colorants such as toner and ink.
Additionally, as the color subtraction model requires the basic colorants to absorb short (blue), medium (green) and long (red) wavelength portions of the visible spectrum similar to the human visual system, the basic primary colors for any printing process are perceived by human as cyan, magenta and yellow. When density is created by combining these colors, it can be difficult to provide additional levels of density without creating an unintended color shift.
To address the color shifting problem and to reduce the amount of toner required to form an image, a black colorant is used. The black toner absorbs the full range of visible spectrum and therefore allows greater color stability and reduced colorant consumption when higher densities are required. Other supplemental colorants, such as orange, violet, light cyan, light magenta and light black, have also been used to extend the printing capabilities to a larger color gamut and to achieve higher quality prints. Nonetheless, all reflective colorants obey the color subtraction model and ultimately the incremental amounts of toner required to achieve visible density gains in the high density becomes prohibitive because of cost limitations or limitations on the ability of a toner printer to pattern, transfer or fuse such amounts of toner to a receiver.
Alternatively, there are examples of efforts to increase the low density limits of dynamic range by improving the brightness of receivers used in toner printing. For example, many cellulosic or paper fibers have a naturally yellow color, and even after bleaching, the fibers in such papers can limit the overall brightness of the receiver and therefore the overall dynamic range of a print made thereon. It has therefore been known to add fluorescing colorants in the form of optical brighteners to such receivers. These optical brighteners absorb invisible wavelengths of an ambient light and use the absorbed energy from these wavelengths to emit a visible blue light. The visible blue light tends to combine with the yellow light to provide a brighter receiver.
As a practical matter, it has been found that it is typically necessary to add significant amounts of optical brightener to a receiver to achieve brightness gains that are generally stable over time. This can occur in some instances because the content of the paper fibers and the environment in which the receiver materials are used or stored can cause an increase in the yellow content of the receiver fiber over time and the optical brighteners are added in amounts that are intended to offset the potential long term yellowing of the receiver fibers. This causes a variety of color variations. For example, when human skin tones are printed on receivers having such high levels of optical brighteners, these unwanted blue emissions give the skin tones a bluish hue. The excessive blue hue in human skin tones can appear to be an unnatural coloration and will be objectionable to most viewers as the reproductions of the original image will be flawed. It will also be appreciated that such optical brighteners add cost to the receiver and must be added to such receivers in a uniform manner such that the cost of receivers having high loadings of optical brighteners can be significant.
Additionally, in the case of toner printing it is possible for the binder to absorb wavelengths of light that are typically absorbed by optical brighteners and re-emitted by the optical brighteners that are typically used in a receiver. Accordingly, optical brighteners on a portion of a receiver that is covered by a clear fused toner mass will re-emit a lower intensity of light than optical brighteners that are not covered by the receiver. This creates a variation in density in a print between toner covered portions and uncovered portions that can lower apparent image quality and create unrealistic artifacts.
Similarly, the emissivity of the fluorescent materials in a receiver can make it difficult to provide natural transitions between areas having higher levels of fluorescent material and areas having lower levels of fluorescent material.
It is also known to use fluorescent toners having fluorescent colorants for security printing purposes and to provide spot colors. For example, U.S. Pat. No. 3,713,861 (Sharp) describes coating a fluorescent material over a document image for anti-copying purposes. It has also been proposed to incorporate fluorescing pigments or dyes into liquid toner particles as described in U.S. Pat. No. 5,105,451 (Lubinsky et al.). Additionally, U.S. Patent Application Publication 2010/0164218 (Schulze-Hagenest et al.) describes the use of substantially clear (colorless) fluorescent toner particles in printing methods over color toner images. Such clear fluorescent toner particles can be used for security purposes since they are not colored except when excited with appropriate light. Other invisible fluorescent pigments for toner images are described in U.S. Pat. No. 6,664,017 (Patel et al.).
Printing processes for providing one or more color toner images are known, but it is also desired that fluorescing effects can also be provided for any type of color toner image in order to expand the color gamut while using conventional non-fluorescing color toners. However, it has been difficult to properly design desired fluorescing effects using known fluorescing colorants (dyes and pigments) as many of them are very sensitive to the illuminating radiation. Further, color reproduction using fluorescing color toners produces unrealistically “bright” colors for most objects. This is usually an undesirable effect. For example, when an illuminating light has some portion of the electromagnetic spectrum that is absorbed by fluorescing colorants that emit at a different wavelength, the overall resulting emissions are very “bright” and may overwhelm the non-fluorescing traditional colors in the color toner images. This again results in unrealistic images.
Additionally, where fluorescent colorants are used in toners, it can be difficult to form toner images having high density high gamma image portions.
Accordingly, there remains a need for toner printing systems and methods that can form toner images with enhanced dynamic range in an efficient manner without creating additional image artifacts.